The invention relates generally to the field of voltage waveform generators and more specifically to the generation of a voltage waveforms across a capacitive load.
Electroluminescent (EL) devices require an AC drive signal in order to produce illumination. The frequency of the drive signal is typically in the audio frequency range. As a result, audible noise can be generated. Devices such as portable telephones which operate in the audio frequency range and have EL lamps can be adversely affected by the EL hum or background noise. Similarly, laptop computers with EL displays can exhibit undesirable background noise.
Decreased power consumption is realized for waveforms that approximate a sinusoid at the EL drive frequency. Drive circuits which generate square waveforms or saw-tooth waveforms can be used to drive the EL lamp to reduce power consumption and audible noise. These circuits typically use thyristors or high voltage metal oxide semiconductor field effect transistors (MOSFETs) biased in the triode region as switching devices, however, this voltage control mode does not allow for accurate control of the EL lamp charge and discharge rates.
This invention relates to a method and circuit for generating a voltage waveform across a capacitive load. A plurality of switches are used to control the current through the capacitive load. Synchronized operation of the switches allows the capacitive load to be linearly charged and discharged. The circuit can be used to generate a trapezoidal waveform for driving an electroluminescent device. The trapezoidal waveform reduces audible noise which can be detrimental in audio devices having electroluminescent displays, such as portable telephones and laptop computers. In addition, the circuit and electroluminescent device achieve a higher power efficiency.
The method includes the steps of charging a reactive load with a substantially constant current, terminating the substantially constant current and discharging the capacitive load to generate a substantially constant current. In another embodiment, the method includes the steps of closing a first switch between a first terminal of the capacitive load and a first terminal of a substantially constant current source and closing a second switch between a second terminal of the capacitive load and a second terminal of the substantially constant current source so that the capacitive load is charged at a substantially linear rate. The embodiment includes the additional steps of opening the first switch, opening the second switch and closing a third switch between the first and second terminals of the capacitive load so that the capacitive load is discharged at a substantially linear rate. In another embodiment, the substantially linear charge rate and the substantially linear discharge rate are approximately equal.
The circuit includes a first switch between the first terminal of the capacitive load and the first terminal of the substantially constant current source, a second switch between the second terminal of the capacitive load and the second terminal of the substantially constant current source and a third switch between the first terminal of the capacitive load and the second terminal of the substantially constant current source. A substantially constant current linearly charges the capacitive load when the first and second switches are closed and the third switch is open. A substantially constant current is generated by the linear discharge of said capacitive load when the first and second switches are open and the third switch is closed.